Detection and warning of ice formation on surfaces is significant for improving the safety and operation of airplanes, unmanned aerial vehicles (UAVs), space vehicles, other motor vehicles (e.g., cars and trains), and structures (e.g., bridges and buildings) in harsh weather conditions. For example, UAVs operate in regions where icing conditions can occur suddenly and cause the aircraft to fail. Conventional techniques for mitigating wing-icing conditions are often not feasible for UAVs due to power and weight constraints. Having the ability to detect an icing condition and navigate away from the area can be important for preserving the UAV.
There are several methods currently used or proposed for the detection of ice formation on airplanes and UAVs. The first method is based on a mass-spring system. The principle of this method is that the resonant frequency of a solid body will alter with a change in mass and stiffness. A piezoelectric material at its natural frequency excites the sensor diaphragm and, as ice forms on the sensor's surface, a change in stiffness occurs, causing the natural frequency to increase. The increase of the natural frequency can be used to indicate ice formation on the sensor surface, and provides a warning for ice formation on the surfaces of airplanes and UAVs. One difficulty with this method is that ice has a tendency to form around and over the mass-spring sensor rather than on the sensor itself, so that the warning signal for the ice formation provided by the sensor occurs later than the time that ice has formed on the target surfaces surrounding the sensor. This delay could result in a serious safety and operation problem for navigating UAVs and airplanes.
Another method is a passive near-infrared reflection device that crews use on the ground to detect ice formation on airplane surfaces from a distance. The method is used before the airplane takes off to determine deicing needs. The method uses a few narrow band pass devices in the 1-μm to 1.5-μm range to detect the presence of ice by measuring the amount of light reflected from the airplane. The system compares the relative intensity of light before it reaches the target and as it returns. The detection of ice formation is based on the difference in reflection intensity from the airplane surface, with and without ice. An incandescent light enables nighttime detection. Crews watch a monitor that shows gray-scale images of the aircraft, with icy areas showing up in red. The system can detect ice layers of 0.5 mm or thicker from as far as 65 feet away. One difficulty with this method is that it is only an on-ground technique and cannot currently be used for navigating airplanes and UAVs.
Another method, described in U.S. Pat. No. 5,929,443, incorporated herein by reference, uses difference diffusive reflectometry and an optical polarization imaging unit to illuminate an airplane and scan a light beam on the airplane wings. Polarization images are then recorded and used for ice detection outside the plane. This technique utilizes the difference in the depolarizaiion of polarized light scattered or diffusely reflected by metal and ice surfaces. One limitation of this technique is that the unit is used to either image an entire airplane or to scan the light beam point-by-point on wings of an airplane from outside the airplane. This is not an on-board application, and the unit is not placed inside the airplane. Harsh weather conditions (e.g., snow, heavy rain, fog, or hail) may also cause interference because the airplanes and the optical detection unit are separated.